I just used the phrase, "Thanks for the heads up" in an email. And it made me think. It warns me about reading poetry in translation. The phrase, of course, comes from ball games--when a wild ball is lobbed, one yells "heads up" so people will look at where the ball is and get out of the way. But I wonder, if the phrase was used in poetry, how would one translate it meaningfully into another language? "Thanks for the warning" doesn't convey the same thing at all.

What does this mean about idiomatic expressions in general?